1. Field of the Invention
This invention relates to a method for replacing an intervertebral disc nucleus. More particularly, the invention relates to a prosthetic device used to position a gel material within the disc nucleus.
2. Description of the Prior Art
The intervertebral disc is a complex joint composed of three component structures: the nucleus pulposus, the annulus fibrosis and the vertebral end-plate. These components work to absorb the shock, stress and motion imparted to the human vertebrae.
The nucleus pulposus occupies 25-40% of the total disc cross-sectional area. It is composed mainly of mucoid material primarily containing proteoglycans with a small amount of collagen. The nucleus pulposus is a loose, amorphous hydrogel having the capacity to bind water. In fact, the nucleus pulposus usually contains 70-90 percent water by weight.
The annulus fibrosis maintains the nucleus pulposus within the center of an intervertebral disc. It is composed of highly structured collagen fibers embedded in an amorphous based substance, also composed of water and proteoglycans.
The two vertebral end-plates are composed of hyalin cartilage, which is a clear "glassy" tissue. The vertebral end-plates separate the disc from adjacent vertebral bodies. This layer acts as a transition zone between the hard, bony vertebral bodies and the soft disc. Because the intervertebral disc is a vascular, most nutrients needed by the disc for metabolism are transported to the disc by diffusion through the end plate area.
The intervertebral joint exhibits both elastic and viscus behavior. Hence, during the application of a load to the disc, there is an immediate distortion or deformation of the disc, often referred to as "instantaneous deformation". Because the natural nucleus of the disc is in the form of a loose hydrogel, which can be deformed easily, the extent of deformation of the disc is largely dependant on the extensibility of the annulus fibrosis. Without the constraint from the annulus fibrosis, bulging of the nucleus pulposus would increase considerably.
Because of the constant pressures applied to the intervertebral disc, the disc often degenerates and may ultimately need medical attention. Specifically, the permanent degenerative processes lead to an impairment in blood supply of the disc tissue with subsequent degeneration, dehydration and loss of elastic properties. There are currently three types of treatment used for treating lower back pain caused by injured or degenerated discs: conservative care, laminectomy and fusion. Each of these treatments has its advantages and limitations. The vast majority of patients with lower back pain, especially those with first-time episodes of back pain, will get better with conservative care treatment.
However, it is not necessarily true that conservative care is the most efficient and economical way to solve the lower back pain problem. Laminectomy usually gives excellent short-term results in relieving the clinical symptoms by removing the herniated disc material (usually the nucleus), which is causing the lower back pain either by compressing the spinal nerve or by chemical irritation. However, a laminectomy is not desirable from a bio-mechanical point of view. In the healthy disc, the nucleus takes most of the compressional loading. However, this load is distributed more into the annulus fibrosis ring when the disc degenerates, causing tearing and delimitation. Removal of the nucleus pulposus in a laminectomy causes the load to be distributed further into the annulus fibrosis ring, which narrows the disc spaces. Fusion generally does a good job in eliminating symptoms and stabilizing the joint. However, because the motion of the fused segment is restricted, fusion increases the range of motion of the adjoining vertebral disc. Possibly enhancing the degenerative process.
In view of the short-comings of the prior back treatments, a method and apparatus for treating intervertebral disc problems is needed. The present invention provides such a method and apparatus.